In order to increase data throughput without the installation of new fiberoptic links, Wavelength Division Multiplexing (WDM) systems are being deployed. These systems rely on the tunability of semiconductor lasers to access a wider portion of the spectrum that optical fibers propagate. This can result in an increase in data rates by an order of magnitude or more.
Standards have been promulgated for the channel spacings in these WDM systems. The frequency spacings are tight for optical frequencies. For laser diodes operating at around 1.5 micrometers (xcexcm), it is typically 100 gigaHertz (GHz); this translates to an approximately 0.8 nanometers (nm) minimum wavelength channel spacing. Newer standards are emerging, which, in some cases, have even tighter channel slots.
How the laser diodes are tuned to operate in the various channels of the WDM systems depends on the specific types of laser diodes used. The wavelength of distributed feedback (DFB) laser diodes is tuned by changing the temperature of the diodes. Temperature control is typically implemented in the context of laser communication modules with a thermoelectric cooler. These devices extract heat using Peltier effect in a manner that can be electrically modulated. Fabry-Perot lasers are similarly tuned by controlling their temperature. Distributed Bragg (DBR) lasers are tuned by temperature and injection current. Tuning across broader ranges is accomplished by varying the laser diode""s structure, such as by changing grating frequency.
Solutions have been proposed for maintaining the proper channel spacing in these WDM systems. The proposals typically rely on precise factory calibration, due to inherent manufacturing variability in distributed feedback lasers, for example, and manual fine tuning of the WDM module wavelength after installation to detect and adjust for wavelength shifts from aging and environmental effects.
The present invention concerns wavelength feedback control and/or monitoring for laser diode systems. As such, it is particularly applicable to maintaining the tight channel spacings found in WDM systems. The invention is unique in its reliance on spatially variable filter material to determine or monitor the laser diode""s wavelength. This class of light filter provides specific advantages based upon its compact and tunable nature, which facilitates the implementation of WDM devices in small well-regulated modules.
In general, according to one aspect, the invention features a feedback controlled laser communication device. As is common, the device comprises a laser diode or laser amplifier that is modulated or the output of which is modulated in response to an input signal to generate an optical signal, encoding the input signal. Spatially variable filter material, however, is arranged to receive at least a portion of the optical signal generated by the laser device, and a detector can be used to detect the thus filtered optical signal. Monitoring and possibly control circuitry then use the response of the detector(s) to thus determine wavelength and potentially provide feedback control.
In a preferred embodiment, the spatially variable filter material has a spatially varying passband. Alternatively, spatially varying low pass, high pass, or narrow bandpass notch filter material could be substituted as well as transmissive or reflective filter material.
Further, the detectors are not strictly necessary. The variable filter material can be used to provide narrow frequency feedback into an amplifier for tunable narrow frequency operation.
Also in the preferred embodiment, the spatially variable filter material and at least one detector are arranged to filter and detect light from a rear facet of the laser diode or amplifier. In this way, the total usable power output of the device is not reduced, essentially relying on the free rear facet light, in the case of the diode. In other implementations, however, light from the front facet could be sampled on a partial or periodic basis or during factory calibration.
In a first embodiment, at least two detectors are actually used. This configuration enables the detection of light above and below, respectively, an assigned center wavelength for the device. The control circuitry tunes the wavelength of the laser diode to maintain a predetermined relationship between magnitudes of the responses from the detectors. Moreover, the power output of the laser diode can be modulated or controlled in dependence on the combined responses of the detectors.
This embodiment provides certain ease and flexibility in manufacturing the devices. The placement of the two detectors will determine the response cross-over point in the filter output. The location of the cross-over wavelength is adjusted by moving either detector with respect to the other, such that the response increases and crossover wavelength changes, or both equally with respect to the midpoint, such that the response increases but crossover wavelength is static.
Two detectors, however, are not necessary. In a second embodiment, a single detector is arranged relative to the spatially variable filter material so that its active area changes spatially with the filter. The wavelength is then controlled to maintain a predetermined response from the detector.
In another single detector embodiment, a linear charge coupled device is used. Each element represents a single wavelength bin.
According to another aspect, the invention also features a wavelength-division multiplexed laser diode communication system, which comprises a plurality of channel subsystems. Each subsystem uses the spatially variable filter material in the wavelength feedback control scheme.
In some implementations of the system, the spatially variable filter material is shared between subsystems such that a single spatially variable filter is used for multiple subsystems. Moreover, detector arrays such as charge coupled devices may be used in control schemes of multiple subsystems.
In general, according to still other aspects, the invention relates to a method for controlling laser diode communication system using spatially variable filter material. Additionally, the material may be used to calibrate wavelength-division multiplexed optical communication systems.
The above and other features of the invention including various novel details of construction and combinations of parts, and other advantages, will now be more particularly described with reference to the accompanying drawings and pointed out in the claims. It will be understood that the particular method and device embodying the invention are shown by way of illustration and not as a limitation of the invention. The principles and features of this invention may be employed in various and numerous embodiments without departing from the scope of the invention.